The future of wind shear: will it decrease the number of hurricanes?

Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? Several modeling studies are now predicting this, and it is a reasonable hypothesis. The most recent study, "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", was published Sunday in Nature Geosciences. The authors, led by Tom Knutson of NOAA's GFDL laboratory, showed that global warming may reduce the number of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century. However, their model also found that the strongest hurricanes would get stronger.

An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

The main sources wind shear over the tropical Atlantic:1) The jet stream is the primary year-round source of high wind shear over the Atlantic. The jet can have two branches--the main northerly polar jet, and a weaker subtropical jet that blows over the Gulf of Mexico or Caribbean. In winter, the jet stream is far to the south, bringing very high levels of wind shear to the tropical Atlantic. The Caribbean Sea is warm enough year-round to support hurricane formation, but high levels of wind shear from the southerly position of the jet stream prevents wintertime hurricanes from forming. In the summer, the jet stream retreats to the north, but can still loop far enough south to create hurricane-hazardous wind shear.

2) The large-scale tropical atmospheric circulation pattern known as the Walker Circulation (Figure 1) can bring high wind shear to the Atlantic. A weak Walker Circulation brings high wind shear, while a strong Walker Circulation--rising air over the tropics near Australia, combined with sinking air of the coast of South America near Peru--brings weak upper-level winds over the Atlantic, resulting in low levels of wind shear.

3) The presence or absence of an El Niño event has a critical impact on wind shear levels. El Niño events weaken the Walker Circulation, bringing strong upper-level winds out of the west to the Atlantic, creating high wind shear.

4) In summer and fall, Tropical Upper Tropospheric Troughs (TUTTs) and upper-level cold-core low pressure systems ("cold lows") that are cut off from the jet stream often wander through the tropics, bringing high wind shear with them.

5) A strong east-to-west flowing jet of air is frequently found at the southern boundary of the Saharan Air Layer (SAL), a hot, dry region of air found off the coast of Africa during hurricane season. This easterly jet often is strong enough to cause significant wind shear over the hurricane development region of the tropical Atlantic.

Figure 1. Schematic drawing of the Pacific Ocean's Walker Circulation. Warm ocean waters over the Western Pacific near Australia heat the air above, causing it to rise. When the rising air reaches the top of the troposphere, it can't rise any further, and is forced to flow eastwards towards the Atlantic. This air then sinks back to the surface near the Pacific coast of South America, then flows back towards Australia as easterly trade winds. Image credit: Wikipedia.

The future of wind shearIn their 2007 paper, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", Gabe Vecchi of NOAA's GFDL laboratory and Brian Soden of the University of Miami looked at 18 of the models used to formulate the "official word" on the science of climate change, the 2007 Intergovernmental Panel on Climate Change (IPCC) climate report. Vecchi and Soden found that in the scenario where CO2 doubles to 720 ppm by year 2100 (the so-called "A1B" scenario), these models predict a 1.5-3.5°C increase in global surface air temperature. However, in the Caribbean and some surrounding regions, at least 13 of the 18 models predict that the amount of wind shear rises by 1-2 mph per degree C of warming (Figure 2). The shear increases largely as a result of a weakening of the Walker Circulation. This weakening brings strong upper-level westerly winds to the Eastern Pacific and Caribbean.

The implicationsIf true, Vecchi and Soden's results imply that we may see fewer hurricanes in the Atlantic and Eastern Pacific by the end of the century, since wind shear is such an important ingredient in their formation. How reliable are these model predictions? If global warming is expected to cause a slowdown in the Walker Circulation and increased wind shear over the tropical Atlantic, shouldn't we be able to see these effects already? There is some evidence that we are seeing these effects. According an article by the same authors published in 2006 in Nature, the observed 0.5-0.6°C global warming in the past century has caused the Walker Circulation to slow down by 3.5%--in line with what theory predicts. Moreover, Wang and Lee (2008) documented a 3 mph increase in wind shear over the tropical Atlantic between 1949-2006 (despite some rather low shear years recently, such as during the record-breaking Hurricane Season of 2005). These results, plus the fact that 13 of the 18 IPCC models predict a tropical Atlantic wind shear increase in the coming century, make the hypothesis that we may see increased wind shear over the Atlantic in coming decades a reasonable one. However, climate scientists Ray Pierrehumbert and Rasmus Benestad argue in a 2006 post on realclimate.org that we need another ten years of observations of the Walker Circulation to confirm that we really are seeing a slowdown. In addition, we need to see if the model predictions of increased wind shear hold up when improved simulations with better data and higher resolutions are performed. These models are fairly primitive in their abilities to simulate these sort of regional climate shifts, and some models predict a strengthening of the Walker Circulation in coming decades--the opposite of what Vecchi and Soden found.

Figure 2. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.

CaveatsAll other things remaining constant, an increase in wind shear will cause fewer hurricanes to form. However, all other things will not remain constant. As the climate warms, Sea Surface Temperatures (SSTs) will warm, which may partially or completely offset the effects of increased wind shear. Vecchi and Soden's research also show a substantial increase in wind shear over most of the Southern Hemisphere's hurricane breeding grounds during their hurricane season, but a significant decrease in wind shear over the Western Pacific and North Indian Oceans. Typhoons and cyclones in these ocean basins may well get more numerous and stronger in the future as a result of the lower wind shear. Much more research remains to be done, and it is far too early to be confident of how wind shear might change in a warming world.

Summary: Pacific climate patterns generally neutral Pacific climate patterns remain generally neutral, although some aspects of the 2007/08 La Niña persist. Lingering La Niña signals include the continued presence of cooler than average water on the equator near the date-line, accompanied by reduced cloud and, in the first few weeks of May, enhanced Trade Winds.

Overall though, the tropical Pacific has been warming gradually during the past few months, with ocean temperatures now near-normal in central and eastern areas. In addition, the Trade Winds have decreased to near-normal levels in the western half of the Pacific and, confirming the return to neutral conditions, the Southern Oscillation Index (SOI) is about zero for the past 30 days.

Computer model predictions show Pacific temperatures continuing to gradually increase over the next two seasons, but remaining near-average. The models indicate a low chance of either a stronger warming to El Niño levels or a re-intensification of La Niña conditions during 2008

These probability parameters usually perform well. Until I see some green or "warmer" I shall not get too excited about any early-season blobs...except the cup full before me this joint is peddling as coffee.

489. StormJunkie 10:03 PM GMT on May 21, 2008 483.lol :~) The wit and humor here just keeps on coming! You are on a roll today Drak. Granted a little bit of a dry one, but if there are three things that I enjoy dry they are Ribs, Martini's, and humor!

483. lol :~) The wit and humor here just keeps on coming! You are on a roll today Drak. Granted a little bit of a dry one, but if there are three things that I enjoy dry they are Ribs, Martini's, and humor!

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL AMERICA...THE GULF OF MEXICO...THE CARIBBEAN SEA...NORTHERN SECTIONS OF SOUTH AMERICA...AND THE ATLANTIC OCEAN TO THE AFRICAN COAST FROM THE EQUATOR TO 32N. THE FOLLOWINGINFORMATION IS BASED ON SATELLITE IMAGERY...METEOROLOGICAL ANALYSIS...WEATHER OBSERVATIONS...AND RADAR.

A TROPICAL WAVE IS ALONG 20W/21W S OF 12N MOVING W 10 KT. VISIBLE IMAGES SHOW A WELL DEFINED BUT BROAD AREA OF LOW-LEVEL CYCLONIC TURNING CENTERED NEAR 6N. SCATTERED SHOWERS AND TSTMS ARE MAINLY WITHIN 300 NM E OF THE AXIS S OF 8N.

A TROPICAL WAVE IS ALONG 52W S OF 10N MOVING W 10-15 KT. VISIBLE IMAGERY REVEALS MODERATE LOW-LEVEL CYCLONIC TURNING ALONG THE WAVE AXIS. HOWEVER...MOST OF ITS DEEP CLOUDINESS HAS BEEN ABSORBED BY A HIGH AMPLITUDE UPPER TROUGH WHICH IS NOW E OF THE WAVE.

TROPICAL WAVE IS ALONG 77W S OF 13N MOVING W 10-15 KT. THIS WAVE IS MAINLY INLAND OVER W COLOMBIA. WHILE ONLY A WEAK SIGNATURE OF THIS WAVE IS APPARENT ON SATELLITE IMAGERY...THE MIMIC TPW ANIMATION DOES SHOW FAIRLY GOOD CONTINUITY OF THIS FEATURE. THIS WAVE MAY BE ENHANCING CONVECTION OVER THE SW CARIBBEAN. SEE CARIBBEAN DISCUSSION FOR MORE DETAILS.

An impressive tropical wave is located just off the coast of West Africa. The wave lies under favorable upper winds and over warm sea surface temperatures. However, future development is not expected to occur because of dry air and unfavorable upper level winds ahead of this feature. This is the 6th wave of the season.

Well, I'm not going to understand any of the model stuff until I have enough uninterupted time to watch the videos... and then I probably won't really get it until I play around with them. So, I'm useless for now.

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The PSU/NCAR mesoscale model (known as MM5) is a limited-area, nonhydrostatic, terrain-following sigma-coordinate model designed to simulate or predict mesoscale atmospheric circulation. The model is supported by several pre- and post-processing programs, which are referred to collectively as the MM5 modeling system. The MM5 modeling system software is mostly written in Fortran, and has been developed at Penn State and NCAR as a community mesoscale model with contributions from users worldwide.